SIPBRANDY Working Group A. Johnston
Internet-Draft Rowan University
Intended status: Informational B. Aboba
Expires: March 22, 2018 Microsoft
A. Hutton
Unify / Atos
R. Jesske
Deutsche Telekom
T. Stach
Unaffiliated
September 18, 2017
An Opportunistic Approach for Secure Real-time Transport Protocol
(OSRTP)
draft-ietf-sipbrandy-osrtp-03
Abstract
Opportunistic Secure Real-time Transport Protocol (OSRTP) is an
implementation of the Opportunistic Security mechanism, as defined in
RFC 7435, applied to Real-time Transport Protocol (RTP). OSRTP
allows encrypted media to be used in environments where support for
encryption is not known in advance, and not required. OSRTP does not
require SDP extensions or features and is fully backwards compatible
with existing implementations using encrypted and authenticated media
and implementations that do not encrypt or authenticate media
packets. OSRTP is not specific to any key management technique for
SRTP. OSRTP is a transitional approach useful for migrating existing
deployments of real-time communications to a fully encrypted and
authenticated state.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 22, 2018.
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Applicability Statement . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Definition of Opportunistic Security for SRTP . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. Implementation Status . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Opportunistic Security [RFC7435] (OS) is an approach to security that
defines a third mode for security between "cleartext" and
"comprehensive protection" that allows encryption and authentication
to be used if supported but will not result in failures if it is not
supported. In terms of secure media, cleartext is RTP [RFC3550]
media which is negotiated with the RTP/AVP (Audio Video Profile)
profile defined [RFC3551]. Comprehensive protection is Secure RTP
[RFC3711], negotiated with a secure profile, such as SAVP or SAVPF
[RFC5124]. OSRTP allows SRTP to be negotiated with the RTP/AVP
profile, with fallback to RTP if SRTP is not supported.
There have been some extensions to SDP to allow profiles to be
negotiated such as SDP Capabilities Negotiation (capneg) [RFC5939] .
However, these approaches are complex and have very limited
deployment in communication systems. Other key management protocols
for SRTP have been developed which by design use OS, such as ZRTP
[RFC6189]. This approach for OSRTP is based on
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[I-D.kaplan-mmusic-best-effort-srtp] where it was called "best effort
SRTP". [I-D.kaplan-mmusic-best-effort-srtp] has a full discussion of
the motivation and requirements for opportunistic secure media.
OSRTP uses the presence of SRTP keying-related attributes in an SDP
offer to indicate support for opportunistic secure media. The
presence of SRTP keying-related attributes in the SDP answer
indicates that the other party also supports OSRTP and encrypted and
authenticated media will be used. OSRTP requires no additional
extensions to SDP or new attributes and is defined independently of
the key agreement mechanism used. OSRTP is only usable when media is
negotiated using the Offer/Answer protocol [RFC3264].
1.1. Applicability Statement
OSRTP is a transitional approach that provides a migration path from
unencrypted communication (RTP) to fully encrypted communication
(SRTP). It is only to be used in existing deployments which are
attempting to transition to fully secure communications. New
applications and new deployments will not use OSRTP.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
3. Definition of Opportunistic Security for SRTP
To indicate support for OSRTP in an SDP offer, the offerer uses the
RTP/AVP profile [RFC3551] but includes SRTP keying attributes. OSRTP
is not specific to any key management technique for SRTP. For
example:
If the offerer supports DTLS-SRTP key agreement [RFC5763], then an
a=fingerprint attribute will be present, or
If the offerer supports SDP Security Descriptions key agreement
[RFC4568], then an a=crypto attribute will be present, or
If the offerer supports ZRTP key agreement [RFC6189], then an
a=zrtp-hash attribute will be present.
To accept OSRTP, an answerer receiving an offer indicating support
for OSRTP generates an SDP answer containing SRTP keying attributes
which match one of the keying methods in the offer. The answer MUST
NOT contain attributes from more than one keying method, even if the
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offer contained multiple keying method attributes. The selected SRTP
key management approach is followed and SRTP media is used for this
session. If the SRTP key management fails for any reason, the media
session MUST fail. To decline OSRTP, the answerer generates an SDP
answer omitting SRTP keying attributes, and the media session
proceeds with RTP with no encryption or authentication used.
If the offerer of OSRTP receives an SDP answer which does not contain
SRTP keying attributes, then the media session proceeds with RTP. If
the SDP answer contains the RTP/AVP profile with SRTP keying
attributes or the SAVP (or UDP/TLS/RTP/SAVP(F)) profile with SRTP
keying attributes, then that particular SRTP key management approach
is followed and SRTP media is used for this session. If the SRTP key
management fails, the media session MUST fail.
It is important to note that OSRTP makes no changes, and has no
effect on media sessions in which the offer contains a secure profile
of RTP, such as SAVP or SAVPF. As discussed in [RFC7435], this is
the "comprehensive protection" for media mode.
4. Security Considerations
The security considerations of [RFC7435] apply to OSRTP, as well as
the security considerations of the particular SRTP key agreement
approach used. However, the authentication requirements of a
particular SRTP key agreement approach are relaxed when that key
agreement is used with OSRTP. For example:
For DTLS-SRTP key agreement [RFC5763], an authenticated signaling
channel does not need to be used with OSRTP if it is not
available.
For SDP Security Descriptions key agreement [RFC4568], an
authenticated signaling channel does not need to be used with
OSRTP if it is not available, although an encrypted signaling
channel must still be used. The use of SDP Security Descriptions
using the RTP/AVP profile is defined in
[I-D.mmusic-opportunistic-negotiation].
For ZRTP key agreement [RFC6189], the security considerations are
unchanged, since ZRTP does not rely on the security of the
signaling channel.
As discussed in [RFC7435], OSRTP is used in cases where support for
encryption by the other party is not known in advance, and not
required. For cases where it is known that the other party supports
SRTP or SRTP needs to be used, OSRTP MUST NOT be used. Instead, a
secure profile of RTP is used in the offer.
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5. Implementation Status
Note to RFC Editor: Please remove this entire section prior to
publication, including the reference to [RFC6982].
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC6982].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC6982], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
There are implementations of [I-D.kaplan-mmusic-best-effort-srtp] in
deployed products by Microsoft and Unify. The IMTC "Best Practices
for SIP Security" document [IMTC-SIP] recommends this approach. The
SIP Forum planned to include support in the SIPconnect 2.0 SIP
trunking recommendation [SIPCONNECT]. There are many deployments of
ZRTP [RFC6189].
6. Acknowledgements
This document is dedicated to our friend and colleague Francois Audet
who is greatly missed in our community. His work on improving
security in SIP and RTP provided the foundation for this work.
Thanks to Eric Rescorla, Martin Thomson, and Richard Barnes for their
comments.
7. References
7.1. Normative References
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[I-D.mmusic-opportunistic-negotiation]
Hutton, A., Jesske, R., Johnston, A., Salgueiro, G., and
B. Aboba, "Negotiating SRTP and RTCP Feedback using the
RTP/AVP Profile", draft-mmusic-opportunistic-
negotiation-00 (work in progress), June 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, .
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC3551, July 2003,
.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006,
.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
2008, .
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, .
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[RFC6189] Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
Media Path Key Agreement for Unicast Secure RTP",
RFC 6189, DOI 10.17487/RFC6189, April 2011,
.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, .
7.2. Informative References
[I-D.kaplan-mmusic-best-effort-srtp]
Audet, F. and H. Kaplan, "Session Description Protocol
(SDP) Offer/Answer Negotiation For Best-Effort Secure
Real-Time Transport Protocol", draft-kaplan-mmusic-best-
effort-srtp-01 (work in progress), October 2006.
[IMTC-SIP]
"Best Practices for SIP Security", IMTC SIP Parity
Group http://www.imtc.org/uc/sip-parity-activity-group/,
2011, .
[RFC5939] Andreasen, F., "Session Description Protocol (SDP)
Capability Negotiation", RFC 5939, DOI 10.17487/RFC5939,
September 2010, .
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
DOI 10.17487/RFC6982, July 2013,
.
[SIPCONNECT]
"SIP-PBX / Service Provider Interoperability SIPconnect
2.0 - Technical Recommendation", SIP Forum http://www.sipf
orum.org/component/option,com_docman/task,doc_download/
gid,838/Itemid,261/, 2017, .
Authors' Addresses
Alan Johnston
Rowan University
Glassboro, NJ
USA
Email: alan.b.johnston@gmail.com
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Bernard Aboba
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
Email: bernard.aboba@gmail.com
Andrew Hutton
Unify / Atos
4 Triton Square
London NW1 3HG
UK
Email: andrew.hutton@atos.net
Roland Jesske
Deutsche Telekom
Heinrich-Hertz-Strasse 3-7
Darmstadt 64295
Germany
Email: R.Jesske@telekom.de
Thomas Stach
Unaffiliated
Email: thomass.stach@gmail.com
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